UnaClocker

I think it'd be a better idea to put the SMT version of the AtMega328 onto a very small daughterboard, with two rows of pins on it, that can plug into the standard Arduino boards. This would allow people to replace the CPU on their Arduino's if it fails, or for any other reason want to change the CPU. It'd be like a breakout board for the CPU. This would have the benefit of there only being one version of the UNO board. It'd have the down side that the CPU's would cost a bit more to make, but with some volume, I bet it would be a negligible difference. Just in case I'm not making any sense, here's an example (albeit in a larger package) of the MegaSquirt 2 CPU that replaced an earlier DIP package CPU.

Unfortunately, I don't think that there is any way that the cost of an entirely separate PCB plus 28 pins for the connector is ever going to be "negligible." There are companies that do this sort of thing in order to provide "backward compatible" parts for chips that have gone "out of production" in their original form, and they manage to charge quite substantial amounts.(There is also the problem that the chip in question here is quite narrow, and actually implementing such a PCB would be substantially difficult.)

UnaClocker

Yeah, I figured the "adapter would have to be longer than the socket, so the CPU that's probably too wide to fit between the rows could be at the end of it (maybe underneath to keep the height down). Well, it was an idea.. Are you the guy from Systm? Or just have his picture as an avatar? I loved that show.

So you're really talking an adapter like thishttp://www.logicalsys.com/painfo-vpasp-vb.asp?adapter=pa-qld6sm18-32f-88but with a couple of pins left off to go from 32 to 28. And narrower, maybe using the pinless package, the 32M1-A, the 328-MU and the 328P-MU, both 5mm wide, while the socket pins are ~8.2mm wide(I don't see what the difference is between 328 & 328P in the datasheet, must be something subtle I am missing).

Um, neither. Maybe everyone looks the same when reduced to avatar-size? (yes, it's actually me. I think this is a big improvement over my last avatar pic, where someone said I looked like someone's grandma!)

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maybe using the pinless package

It would be very tight, but it might JUST work. It looks like the pins are laid out somewhat more conveniently that I expected. OTOH, there's also that exposed pad...

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both 5mm wide, while the socket pins are ~8.2mm wide

5mm, plus a bit for the pads. Where did you get 8.2mm? Socket pins are 0.3 inches; 7.62mm. But that's center-to-center. Subtract about .35mm for the pin diameter, and .2mm for the pad that the pin needs to solder to, on both sides, and you're left with about .75mm on each side of the tiny package. Very tight...

ccfreak2k

So you're really talking an adapter like thishttp://www.logicalsys.com/painfo-vpasp-vb.asp?adapter=pa-qld6sm18-32f-88but with a couple of pins left off to go from 32 to 28. And narrower, maybe using the pinless package, the 32M1-A, the 328-MU and the 328P-MU, both 5mm wide, while the socket pins are ~8.2mm wide(I don't see what the difference is between 328 & 328P in the datasheet, must be something subtle I am missing).

P is the picopower variant; it uses less current and is otherwise 100% compatible.